One type of furnace used in glass processing systems to heat glass sheets for processing includes a horizontal conveyor for conveying glass sheets through the furnace for heating within a heating chamber of the furnace. Two types of horizontal conveyors are conventionally utilized. One type is gas hearth conveyors which convey glass sheets horizontally while supported on a thin film of pressurized gas with the conveyance being provided by a moving frame toward which the plane of conveyance is tilted a very slight extent. The other type is roller conveyors which include rolls spaced along the direction of conveyance such that rotational driving thereof conveys glass sheets supported on the rolls. With both types of conveyors, the furnace conventionally includes a horizontally elongated access opening at both its entrance and exit ends. Such entrance and exit openings to the furnace heating chamber along the plane of conveyance through the furnace must have a sufficient vertical height to allow glass sheets to enter and exit the furnace but should not have an excessively large size as this will permit heat to be unnecessarily lost to the environment.
In flat glass tempering systems including the type of horizontal conveyor described above, a quench unit is conventionally located adjacent the exit opening to provide tempering of heated glass sheets conveyed on a roller conveyor of the quench unit. Upper and lower blastheads of the quench unit oppose each other to provide quenching air that cools the upper and lower surfaces of the heated glass sheet which is conveyed therebetween in order to perform the tempering. One problem usually involved is that a certain amount of the quenching air from the quench unit enters the furnace through the adjacent exit opening. This quenching air not only lowers the temperature of the furnace heating chamber but can also prematurely cool a heated glass sheet just prior to transfer thereof from the furnace to the quench unit. This problem is conventionally referred to in the flat glass tempering industry as "blow-back."
In order to avoid the blow-back problem discussed above, furnaces of prior glass tempering systems have included a pivotal door that is movable to an open position to allow the heated glass sheet to be transferred from the furnace to the quench unit whereupon the door is moved to a closed position to prevent the quenching air from entering the furnace. However, such doors are mounted in a relatively hot environment and it is thus somewhat costly to construct and maintain the door operating mechanism. Furthermore, during transfer of the heated glass sheet from the furnace to the quench unit, quenching air can still flow along the upper surface of the glass sheet into the furnace with the door in its open position.
U.S. Pat. Nos. 3,934,970, 3,947,242, and 3,994,711 each disclose a glass tempering system including a furnace having an upper housing with end walls that define the upper extremities of entrance and exit openings at opposite ends of the furnace. A counterbalance mechanism of the furnace raises and lowers the upper housing and thereby controls the size of the entrance and exit openings. Suitable adjustment of the openings provides a sufficient vertical height so as to allow glass sheets to be conveyed into and out of the furnace while not being so great as to allow excessive heat loss through the openings or an excessive amount of quenching air to enter the furnace through the exit opening. However, raising of the upper housing to accommodate thicker glass at the entrance and exit openings also leaves a gap at each lateral side of the furnace through which heat loss can take place. Furthermore, the end walls are subjected to a relatively high temperature that makes it difficult to maintain straight edges for defining the upper extremities of the entrance and exit openings.